A laser marker is attracting attention as a technology for stamping marks such as a product serial number and an identification sign on a produced semiconductor device in a semiconductor device production process.
Such a laser maker scans a liquid crystal mask having thereon a desired pattern to be marked by a laser beam to mark the pattern on an object such as IC by the laser beam passed through the liquid crystal mask.
Conventionally, when the liquid crystal mask is driven, the ambient temperature at which the liquid crystal is operated is set to about the room temperature in consideration of the temperature characteristic of liquid crystal itself of a liquid crystal mask circuit board and the temperature characteristic of peripheral circuits on the circuit board.
Thus, the conventional technology drives the liquid crystal mask in an operation temperature range of about the room temperature, so that the liquid crystal element has a slow speed of response. Therefore, it takes time to change the patterns to be marked, and a working efficiency of laser marking is not improved satisfactorily. In other words, when the liquid crystal element is driven in the operation temperature range of about the room temperature, a viscosity of the liquid crystal element is increased as compared with its driving at a high temperature, resulting in lowering the speed of response.
Recently, there have been proposed the liquid crystal element and peripheral circuits having good heat-resisting properties, and the environment, where the operation temperature range of the liquid crystal mask can be increased to the room temperature or above without any problem, is being prepared.
FIG. 8 shows a relation between liquid crystal drive voltage V and transmissivity of liquid crystal at five different temperatures. Conventionally, when the liquid crystal mask is driven, the liquid crystal drive voltage (ON voltage) V is set to voltage value Va of a range that the transmissivity of liquid crystal is largely changed in correspondence with a change in liquid crystal temperature. Therefore, the conventional technology adjusts to increase or decrease the drive voltage V depending on the temperature change in order to keep the transmissivity of liquid crystal as constant as possible.
But, the conventional technology cannot be used under a situation that the temperature change is extreme and its control to keep the transmissivity of liquid crystal at a fixed level is limited. And, the conventional technology needs an additional circuit for controlling to keep the transmissivity at a fixed level. Thus, it is disadvantageous in view of a cost and a space.
The present invention was achieved in view of the circumstances described above. And it is an object of the invention to provide a method and apparatus for laser marking which have improved efficiency of laser marking work by improving a response speed of liquid crystal and can perform the laser marking operation without being influenced by a change in external environmental temperature.
It is also an object of the invention to provide a method for driving a liquid crystal element, which can keep a constant transmissivity even under a condition with an extreme change in temperature.